Forms of utilization of a femto base station (Home Node B, hereinbelow abbreviated as “HNB”) in industry that can be considered include, for example, forms of utilization such as a compact wireless base station for residential use and a compact wireless base station in a business.
Offering a service by means of an HNB has the following advantages:    (1) Communication services can be offered in dead zones that are beyond the reach of radio waves of a macro base station.    (2) A charge service can be offered that is less expensive than normal charge services offered by macro base stations.    (3) Because high line quality (Ec/Io) can be obtained by a mobile station when the distance between the base station and the mobile station is small, high-speed technologies such as 64 QAM (64 Quadrature Amplitude Modulation) or MIMO (Multiple Input Multiple Output) can be utilized to enable the provision of high-speed packet service under the command of the HNB.    (4) A special content service can be offered that takes advantage of the locality of the HNB.
Because the services realized by an HNB have many advantages as stated above, services should be offered only to subscribers that have contracted with a communication common carrier or to subscribers that have permission from the owner of the HNB.
Accordingly, CSG (Closed Subscriber Group) was introduced in Release 8 in the 3GPP (3rd Generation Partnership Project) to limit access and reception of services of an HNB to only mobile stations of groups that have permission.
Details regarding CSG are next described with reference to FIG. 1.
The 3rd Generation mobile communication system shown in FIG. 1 includes: HNB 20, femto base station gateway (Home NodeB GW, hereinbelow abbreviated as “HNB-GW) 30, switching station (Mobile Switching Center, hereinbelow abbreviated as “MSC”) 40, packet-switching station (Serving GPRS Support Node, hereinbelow abbreviated as “SGSN”) 50, and 3rd generation-conforming mobile stations 10-2 and 10-2.
Of mobile stations 10-1 and 10-2 that are in range and under the command of HNB 20 in FIG. 1, mobile station 10-1 is an authorized mobile station. In contrast, mobile station 10-2 is a mobile station that attempts to receive service provided by HNB 20 without authorization and is hereinbelow referred to as unauthorized mobile station 10-2. In the following description, “mobile station 10” is used when either mobile station is specified.
HNB 20 is connected to a core network of an operator by way of HNB-GW 30.
The core network includes, as core network apparatuses, MSC 40 that controls circuit switching and SGSN 50 that controls packet switching.
When the CSG function is supported, HNB 20 reports the CSG identifier (CSG identity) of its own CSG cell to mobile station 10 that is in the range and under the command of HNB 20.
Mobile station 10-1 decodes the CSG identifier that was reported from HNB 20 and determines whether the CSG identifier is included in the CSG list held by mobile station 10-1.
If the CSG identifier is included in the CSG list, mobile station 10-1 is able to camp on the CSG cell within which it is in range and then receive various services such as originating calls or taking incoming calls.
On the other hand, if the CSG identifier is not included in the CSG list, mobile station 10-1 does not camp on the CSG cell within which it is in range and implements selection of a suitable CSG cell different from that CSG cell.
By means of this mechanism, HNB 20 can be accessed only by limited mobile stations 10-1 that have the CSG identifier of the CSG cell of that HNB 20.
However, a case can be considered in which an entity such as unauthorized mobile station 10-2 shown in FIG. 1 attempts to receive service without authorization in a CSG cell of HNB 20 for which access is not supposed to be available regardless of whether the CSG function is supported.
In such a case, MSC 40 or SGSN 50 checks the IMSI (International Mobile Subscriber Identity) of mobile station 10 and the CSG identifier of the CSG cell in which mobile terminal 10 is located to implement access regulation for regulating access to HNB 20 by mobile station 10 (3GPP TS25.467 Ver 8.0.0 Section 5.1.3).
On the other hand, because the CSG function is a function introduced by Release 8 of 3GPP, cases will occur in which mobile terminal 10-1 that predates Release 8 does not support the CSG function. Alternatively, cases will also occur in which HNB 20 does not support the CSG function.
In such cases, HNB 20 implements an Identification procedure (3GPP TS24.008 Ver 8.4.0) with respect to mobile station 10-1 in order to inquire regarding the IMSI of mobile terminal 10-1, or HNB 20 implements HNBAP (HNB Application Part): UE REGISTER REQUEST procedure (3GPP TS25.469 Ver 8.0.0) with respect to HNB-GW 30 to register mobile station 10-1 in HNB-GW 30. At this time, HNB-GW 30 carries out access regulation by checking whether the IMSI of mobile station 10-1 is able to access HNB 20.
If HNB-GW 30 determines that mobile station 10-1 is able to access HNB 20, HNB-GW 30 notifies HNB 20 that access is permitted by means of an HNBAP: UE REGISTER ACCEPT message. In this way, service realized by HNB 20 is provided to mobile station 10-1.
On the other hand, if mobile station 10 is unauthorized mobile station 10-2 shown in FIG. 1, the IMSI of unauthorized mobile station 10-2 is not registered to allow access to the CSG. As a result, HNB-GW 30 determines that unauthorized mobile station 10-2 cannot access HNB 20 and notifies HNB 20 by means of an HNBAP: UE REGISTER REJECT message that access is not permitted. In this way, the RRC (Radio Resource Control) connection between unauthorized mobile station 10-2 and HNB 20 is cut (3GPP TS25.467 Ver 8.0.0 Section 5.1.2).
As described hereinabove, when service is offered by means of HNB 20, MSC 40, SGSN 50, or HNB-GW 30 implements access regulation based on the IMSI of mobile station 10, whereby access to HNB 20 is rejected on the mobile communication network side during a signal establishment procedure even if unauthorized mobile station 10-2 that is not permitted to access HNB 20 should originate a call.
In the 3GPP standards, however, the ability to originate a call is prescribed when the call type is an emergency call even for mobile station 10 that is not permitted access to HNB 20 (3GPP TS22.011 Ver 8.6.0 Section 8.5.1).
When the call type is an emergency call, mobile station 10-1 sets “Emergency Call” in an Establishment Cause parameter that indicates the cause of the establishment request in the RRC: RRC CONNECTION REQUEST message or RRC: INITIAL DIRECT TRANSFER that is transmitted to HNB 20 at the time of requesting establishment of an RRC connection or at the time of requesting establishment of a signaling connection (3GPP TS25.331 Ver 8.5.0, Section 10.3.3.11, Patent Document 1).
HNB 20 then sets an “Emergency call” value in the Registration Cause parameter of an HNBAP: UE REGISTER REQUEST message that is transmitted to HNB-GW 30.
When the Registration Cause parameter is “Emergency call,” HNB-GW 30 does not implement access regulation based on the IMSI (3GPP TS25.467 Ver 8.0.0 Section 5.1.2).
By means of this method, even mobile station 10 that is not permitted to access HNB 20 is able to skip the access regulation of HNB-GW 30 and access HNB 20 when the call type is an “Emergency Call.”
The composition of the RRC: RRC CONNECTION REQUEST message is here shown in FIG. 2, the composition of the RRC: INITIAL DIRECT TRANSFER message is shown in FIG. 3, the composition of the Establishment Cause parameter in RRC protocol is shown in FIG. 4, the composition of the HNBAP: UE REGISTER REQUEST message is shown in FIG. 5, and the composition of the Registration Cause parameter in HBNAP protocol is shown in FIG. 6.